In gas separations, it is advantageous to use membranes which possess the desired properties of selectivity, flux, and mechanical strength to withstand and prolong operation at high temperatures and pressures without suffering morphological compaction. In order for gas separations to be commercially viable, it is advantageous to use membranes that can be manufactured in large quantities at high product quality, and which can be inexpensively assembled into a permeator. Membranes which have been found to be particularly advantageous for commercial applications are asymmetric, hollow fiber membranes. These membranes have a thin separating layer integral with a porous substrate that provides support to the separating layer but which offers little, if any, resistance to passage of gases.
Hollow asymmetric fiber membranes that have a separating skin on the exterior of the fiber have a graded density skin, that is, a skin which exhibits maximum density on the exterior of the fiber at the surface which is farthest from the porous substructure. Asymmetric membranes are substantially chemically homogeneous and exhibit selective permeation for at least one gas of a gaseous mixture over that of at least one other gas of that mixture.
Processes for manufacture of asymmetric membranes must be capable of forming high quality membranes. Hollow fiber asymmetric membranes are commonly produced by air-gap spinning. In air-gap spinning, a solution of polymer is extruded through a spinneret suitable for forming the hollow fiber. During spinning of the fiber, a gas or liquid may be injected into the bore of the hollow fiber extrudate to maintain the configuration of the hollow fiber. The resulting hollow fiber extrudate travels through an air-gap prior to coagulation by known techniques such as by contact with a non-solvent for the polymer. The fibers are then collected onto a takeup roll or other suitable collection device.
The hollow fiber spinning process depends on many variables which may affect the morphology and properties of the hollow fiber membrane. These variables include the composition of the polymer solution employed to form the fiber, the composition of fluid injected into the bore of the hollow fiber extrudate during spinning, the coagulation medium employed to treat the hollow fiber extrudate, the rapidity of coagulation of the polymer, the rate of extrusion of the fiber, takeup speed of the fiber onto the takeup roll, and the like.
The hollow fiber membranes formed by the air-gap spinning process of the prior art are generally useful for separating gases. The utility of these membranes, however, may be limited by the decrease in selectivity that occurs as the draw ratio increases. Retention of selectivity while utilizing increased draw ratios would enable the resultant fibers to be produced at lower cost and to thereby find a greater range of applications than those membranes produced by the prior art.